[16.02] Gravitational accretion of particles in Saturn's rings

R. Karjalainen, H. Salo (Univ. of Oulu)

The formation of semi-permanent particle groups in the outer
parts of Saturn's rings is studied via local simulations
including the mutual collisions and gravitational
interactions between particles. Different factors, including
the internal density of particles, their elasticity, and
size distribution, are studied. Special attention is paid
for removing the uncertainties caused by simulation-related
parameters (e.g. number of particles, time step, method of
force calculations). Also, for each distance and particle
properties, typically 20 separate experiments are carried
out, lasting 50 orbital periods. Simulations indicate that
for 1-meter sized identical particles with solid ice density
(\rho = 900 \ kg/m3), and Bridges et al.
velocity-dependent elasticity, (\epsilonB(v)), the
particle groups start forming at distance a = 136 \ 000 \
km. However, the transition from non-accretion to accretion
takes place over a relatively wide transition zone: only
beyond 145 \ 000 \ km are stable groups seen in
practically all separate experiments. For constant \epsilon
= 0.5 the behavior is about the same as with
\epsilonB(v), whereas for \epsilon = 0.1 the transition
zone is moved closer to planet by almost 10 \ 000 \ km's.
In the case of power-law size distribution the transition to
aggregate formation moves inward about 8 \ 000 \ km (for q
= 3, and 0.5 < r < 5 m). For different internal densities
these distances scale with \rho-1/3. The stable groups
seen in simulations have typically retrograde rotation
whereas groups with prograde rotation often dissolve in
about 5 to 10 orbital revolutions. Preliminary studies of
the effect of particle spins and surface friction is also in
progress.

This study is supported by Väisälä foundation and the
Academy of Finland.